The present invention relates to a phosphorescent phosphor, and more particularly, to a novel phosphorescent phosphor which shows excellent photo-resistance required for the phosphorescent phosphor to be utilized both indoors and outdoors mainly as a night-time display, and which shows an extremely long afterglow characteristics.
Generally, the afterglow time of a fluorescent substance is short, i.e., the light emitted from the fluorescent substance decays immediately after removal from the source of excitation. Unlike such a fluorescent substance, some substances emit light after having absorbed ultraviolet radiation or the like and afterglow thereof that can be visually observed continues for a considerable time (ranging from several tens of minutes to several hours) after the source of stimulus is cut off. Such substances are called phosphorescent phosphors.
As phosphorescent phosphors, sulfide phosphorescent phosphors are known. Examples of sulfide phosphorescent phosphors include CaS:Bi (which emits light of violet blue), CaStS:Bi (which emits light of blue), ZnS:Cu (which emits light of green) and ZnCdS:Cu (which emits light of yellow or orange). However, any of these sulfide phosphorescent phosphors is chemically unstable and shows degraded light resistance, i.e., it suffers from problems that must be solved for practical use.
The most extensively used phosphorescent phosphor among such sulfide phosphorescent phosphors is zinc sulfide phosphorescent phosphor (ZnS:Cu). However, zinc sulfide phosphorescent phosphor is decomposed as the result of irradiation by ultraviolet radiation in the presence of moisture and thus blackens or reduces the luminance thereof. Therefore, it is difficult to use this phosphorescent phosphor in fields where it is placed outdoors and exposed to a direct sunlight, that is, application thereof is limited to luminous clocks/watches or clocks/watches and instrument dials, evacuation guiding signs or indoor night-time display.
Even when zinc sulfide phosphorescent phosphor is used for a luminous clock, since the afterglow thereof which allows the time to be visually recognized lasts only from 30 minutes to 2 hours, a radioactive substance must be doped to the phosphorescent phosphor and a self-luminous paint which keeps emitting light by absorbing an energy of radiation from radioactive substance must be employed.